The present invention relates to hydrostatic transaxle assemblies, and more particularly, to such transaxle assemblies of the type including a hydrostatic transmission (HST) and an axle assembly including gearing, a differential, and a pair of axle shafts.
Hydrostatic transaxles of the type to which the Present invention relates have been used commercially on relatively small lawn and garden tractors. For a number of years, it has been known to combine a hydrostatic transmission, such as the Model M-7 HST sold commercially by Eaton Corporation, with a mechanical axle assembly, typically including speed reduction gearing, a differential gear set and a pair of axle shafts. Although such HST-gear axle assemblies have generally operated in a satisfactory manner, the overall configuration of the HST and gear axle assembly has generally not been considered optimum, especially for use on relatively smaller lawn and garden tractors.
More recently, some of those skilled in the art have attempted to combine the HST and the gearing and differential in a single assembly, thus improving the overall configuration (i.e., the size and shape relative to the rest of the tractor) An example of such a device is illustrated in U.S. Pat. No. 4,856,368, in which an axial piston pump and axial piston motor are disposed within a fairly complex assembly of housing portions. Although the device of the '368 patent could have improved the overall configuration of the transaxle, one disadvantage of the particular construction is that the hydraulic fluid utilized by the axial piston pump and motor would be in communication with the fluid or grease used to lubricate the gear train and differential. As is now well known to those skilled in the art, the level of contamination (i.e., small metal or dirt particles) must be kept much lower in a hydrostatic pump and motor than what is acceptable in a gear train or in differential gearing.
In U.S. Pat. No. 4,979,583, there was an attempt to overcome the problem of mixing of fluids by "encapsulating" a complete hydrostatic transmission between a pair of axle casing halves which parted on a horizontal plane containing the axis of rotation of the axle shafts. Furthermore, the stated intent in the '583 patent was to accomplish the encapsulation in a way which would segregate the hydrostatic transmission from the gearing, thus keeping contamination from the gearing out of the hydraulic circuit of the HST.
Although allegedly solving the contamination problem, the device of the '583 patent inherently involved fairly complex structure, in order to achieve the encapsulation of the hydrostatic transmission between the upper and lower axle casing halves. Another disadvantage of the '583 patent relates to periodic servicing of the device. As is well known to those skilled in the art, it is frequently necessary or desirable to service or repair either the hydrostatic transmission or the gearing only. In the device of the '583 patent, it would be necessary to completely disassemble the axle casing halves, in order to have access to the HST, for purposes of performing service or repair.